<!doctype html>
<html>
<head>
<meta charset="utf-8">
<title>Untitled Document</title>
<style>
	#div1 {width: 100px; height: 151px; background: red; position: absolute;right: 0; bottom: 0;}
	#txt1 { position: fixed;right: 0;top: 0; }
</style>
<script>
	window.onscroll = function()
	{
		 var oDiv = document.getElementById('div1');
			var scrollTop = document.documentElement.scrollTop || document.body.scrollTop;
			//oDiv.style.top = (document.documentElement.clientHeight - oDiv.offsetHeight)/2 + scrollTop + 'px';
		 startMove(parseInt((document.documentElement.clientHeight - oDiv.offsetHeight)/2 + scrollTop));
	};
	
	var timer = null;
	function startMove(iTarget)
	{
			document.title =iTarget;
		 var oDiv = document.getElementById('div1');
			clearInterval(timer);
		 timer = setInterval(function(){
					var speed = (iTarget-oDiv.offsetTop)/4;
					speed =speed>0?Math.ceil(speed):Math.floor(speed);
					
					//document.title =oDiv.offsetTop + ',' + speed;
					document.getElementById('txt1').value = oDiv.offsetTop;  //调试用
					if(speed==0)
						{
							clearInterval(timer);
						}else{
							oDiv.style.top=oDiv.offsetTop + speed + 'px';
						}
			},30);
	}
</script>
</head>

<body style="height: 2000px">
<input type="text" id="txt1">
<div id="div1"></div>
</body>
</html>
